Direct Methanol Fuel Cells (DMFC)

A subcategory of PEMFCs, DMFCs use methanol. They are simple, relatively inexpensive fuel cells. DMFCs with power outputs between 25 watts and 5 kilowatts are currently available for purchase. They can run up to 100 hours between refueling.

Function and Reactions of DMFCs

Methanol is injected into the fuel cell along with steam where it reacts with a platinum-ruthenium catalyst to form hydrogen, carbon dioxide, and electrons. At the cathode, hydrogen and oxygen combine to form water.

Benefits of DMFCs

Methanol is an easily transported, reasonably energy dense fuel that makes DMFCs practical for use in transportation. DMFCs avoid may of the problems associated with the transport and storage of hydrogen and are simpler than methanol reformation fuel cells.

Methanol fuel cells have been miniaturized for military and commercial use. Their current top use is as fuel cells for recharging laptop batteries. The energy density of methanol is 15 times greater than that of a lithium-ion battery of the same size, making them ideal for laptops that will be used in remote locations.

Though their voltage is limited for reasons explained below, DMFCs can still carry a great deal of energy to be released slowly over time. This makes them ideal for battery recharging as mentioned above and even for smaller vehicles like forklifts.

Drawbacks of DMFCs

There are two major problems associated with direct methanol fuel cells. The first is that methanol can cross the proton exchange membrane and react with the platinum catalyst on the cathode side of the fuel cell. This means that reactants that should form on the anode side only are also forming on the cathode side. The net result is a reduction in voltage and thus a reduction in the power generated by the fuel cell.

The action of methanol crossing the PEM is called cross-over current. To combat this, the concentration of methanol on the anode side is kept low. The unfortunate side effect of doing this is a limit on the voltages that can be obtained.

The second major problem in this type of fuel cell is the production of carbon monoxide, which poisons the platinum aspect of the catalyst and makes the cell less efficient. The current solution to this is a platinum-ruthenium or platinum-gold hybrid catalyst, which is less affected by carbon monoxide.

In addition to the above mentioned drawbacks, it needs to be pointed out that methanol fuel cells still rely on fossil fuels and still produce greenhouse gases. It is true that the energy gained per quantity of carbon dioxide produced is quite high when compared to internal combustion engines and other methods that simply burn fuels.

One final topic to discuss is the complexity of methanol fuel cells. Because water is injected along with methanol, the water has to be recycled so as not to saturate the proton exchange membrane and reduce efficiency of the cell. Additionally, methanol fuel cells are subject to water drag, which occurs when water is pulled along with protons from the anode to the cathode. This results in a relative overabundance of water on the cathode side and a deficiency on the anode side of the FC. A water recovery loop is necessary to maintain steady operation of a DMFC.